Post on 23-Feb-2016
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Further Reading: Chapter 06 of the text book
- what is a cloud?
- cloud classifications
- clouds and precipitation
Natural Environments: The AtmosphereGE 101 – Spring 2007
Boston University
MyneniLecture 15: Clouds
Feb-23-07(1 of 11)
Introduction
Natural Environments: The AtmosphereGE 101 – Spring 2007
Boston University
MyneniLecture 15: Clouds
Feb-23-07(2 of 11)
• Previously,– We learned that when air is displaced vertically, it cools due to adiabatic processes– Discussed causes of vertical motion
• Convection• Orographic lifting
– We also discussed what happens once the temperature cools to the dew point temperature, namely, we expect clouds to form as water vapor changes to liquid
• Today,– We want to discuss what forms these clouds take
• Classify cloud formations• Discuss characteristics that allow us to identify different types of clouds
Cloud Type by Form
Natural Environments: The AtmosphereGE 101 – Spring 2007
Boston University
MyneniLecture 15: Clouds
Feb-23-07(3 of 11)
• Clouds:– Optically thick mass of suspended water drops or ice crystals
• Clouds can be classified by some simple, but subjective, criteria that also provides information on the atmospheric conditions
• One form of classification is based on appearance or form
Cirrus: Thin wispy clouds Stratus: Layered clouds with fairly continuous coverage
Cumulus: Individual clouds characterized by heaped, puffy appearance
Have vertical extent, from 1-3km up to tropopause
Cloud Type by Altitude-01
Natural Environments: The AtmosphereGE 101 – Spring 2007
Boston University
MyneniLecture 15: Clouds
Feb-23-07(4 of 11)
• Can also classify them based on their altitude• “Cirro”• “Alto”• “Strato”• “Fog” Cirro
High clouds (7-18) kmCold (<-25 C) with exclusively ice crystals
Cirrocumulus: high, puffy cloudsCirrus: high, wispy clouds
Cloud Type by Altitude-02
Natural Environments: The AtmosphereGE 101 – Spring 2007
Boston University
MyneniLecture 15: Clouds
Feb-23-07(5 of 11)
Altostratus: thin, layered cloudsAltocumulus: individual or “rolls” of clouds
AltoMiddle level clouds (2-7 km)
-25 to 0 C composed of both water and ice crystals
Cloud Type by Altitude-03
Natural Environments: The AtmosphereGE 101 – Spring 2007
Boston University
MyneniLecture 15: Clouds
Feb-23-07(6 of 11)
Stratus: Dense, uniform gray layers Stratocumulus: groups of dense, puffy clouds
StratoLow level clouds (0 - 4 km)
> 5 C composed of water
Cloud Type by Altitude-04
Natural Environments: The AtmosphereGE 101 – Spring 2007
Boston University
MyneniLecture 15: Clouds
Feb-23-07(7 of 11)
Fog“Clouds at ground level”
Radiation fog: forms at night when cold ground cools the air above it (in valleys)Advection fog: forms when warm, moist air moves over colder surface and cools
(in coastal areas)
Cloud Type by Rain
Natural Environments: The AtmosphereGE 101 – Spring 2007
Boston University
MyneniLecture 15: Clouds
Feb-23-07(8 of 11)
• Finally, we can classify them based on the presence of rain• Nimbus: any cloud that rains
Cumulonimbus: vertical clouds that produce rain Nimbostratus: low, flat clouds that drizzle
Thunderstorms
Natural Environments: The AtmosphereGE 101 – Spring 2007
Boston University
MyneniLecture 15: Clouds
Feb-23-07(9 of 11)
– Thunderstorms form when we have an unstable, moist atmosphere resulting in strong vertical motions
– Actually composed of many individual circulation “cells”
• Updraft region is where the upward motion is intense and where the rain typically falls
• Downdraft region is associated with downward motion which can also be intense -> leads to downbursts
• Cells are typically 10-20km across and circulation within them occurs over 20 minutes or so, i.e. new cells can form in 20 minutes
– Can produce hail: this is when an ice particle is continuously cycled through the convection cell before becoming heavy enough to fall out
– Can also produce lightening: as water is moved within the cell, it develops a frictional charge; the discharge occurs through a spark, i.e. lightening
Clouds and Precipitation-01
Natural Environments: The AtmosphereGE 101 – Spring 2007
Boston University
MyneniLecture 15: Clouds
Feb-23-07(10 of 11)
• Rain (or any precipitation)– Requires
• Vertical motions -> cooling• Presence of condensation nuclei -> something for the rain drops to form on (for
example a small dust particle)
– Condensation does not necessarily mean rainfall; could just form clouds
– In order to form precipitation, the drops must be large enough to fall against the upward motion of the air
• This requires that the aggregation (or coalescence) of lots of water molecules into drops
• Cloud droplets are typically 50-100 micro-meters in diameter• In contrast, rain drops are typically 500 micro-meters before they begin to fall
(drizzle)• At 1000-2000 micro-meters, they are considered rain drops
Clouds and Precipitation-02
Natural Environments: The AtmosphereGE 101 – Spring 2007
Boston University
MyneniLecture 15: Clouds
Feb-23-07(11 of 11)
• There are also other forms of precipitation• Snow - crystallized water formed by the aggregation of frozen water• Sleet - rain falling through a colder lower layer and freezing• Hail - the aggregation of liquid water onto frozen water and subsequent freezing• Note that precipitation does not necessarily fall straight to the ground - it might start
to fall then get caught in an updraft and cycle through; during this process more aggregation occurs and we get very large rain or hail falling out (i.e. ‘golf-ball size hail’)